1. Field of the Invention
The present invention relates to new 3,6,6-trimethylbicyclo[3.1.0]hexane derivatives and to processes for their preparation. These compounds are useful materials in the preparation of insecticidally active synthetic pyrethroids.
2. Description of the Prior Art
The invention relates to a compound and to a process for its preparation. This compound is a useful intermediate in the preparation of insecticidally active compounds. The latter compounds are of the pyrethrin type and may, therefore, be called "pyrethroids". As these pyrethroids combine exceptionally good insecticidal properties with a very low mammalian toxicity, they are of great interest to the agrochemical industry and considerable effort has been expended in finding economic routes for their production.
The general formula of one class of these pyrethroids described in U.S. Pat. No. 4,024,163 may be represented as follows: ##STR1## where each asterisk denotes an asymmetric carbon atom, each X represents a halogen atom and R is a member of a group of radicals known to impart insecticidal activity to the molecule, for example, 3-phenoxybenzyl or alpha-cyano-3-phenoxybenzyl. It is known that for maximum insecticidal activity the acid part of the ester of formula I should be in the (1R,cis) form, i.e. the absolute configuration at carbon atom 1 is R and the two hydrogen atoms on carbon atoms 1 and 3 are in a cis relationship. This nomenclature is known as the Elliott nomenclature and is defined in M. Elliott et al., Nature, 248(1974)710.
It follows, therefore, that if these stereoisomeric esters of formula I are to be prepared, either a stereospecific chemical route is required or the desired stereoisomer must be obtained from a racemic form by physical separation techniques. The latter are expensive and laborious and are not readily employed on an industrial scale. In a stereo-specific route the naturally occurring substance (+)-3-carene is used, whose formula is as follows: ##STR2## This compound is an inexpensive, readily available natural terpene, which is used as a starting material for the preparation of pyrethroid esters of formula I, via a stereospecific route involving processes disclosed in the present description.